The semiconductor industry is requiring smaller dimensions to make state-of-the-art semiconductor devices. In forming these devices, the limits of optical lithographic techniques are fast approaching. Other advanced lithographic techniques are being investigated to replace optical lithography once resolution and depth of focus limits of the optical lithography tools are reached. One lithographic technique that is being investigated is projection electron beam lithography. One specific implementation is called scattering with angular limitation in project electron beam (E-beam) lithography, which is also called SCALPEL.
FIG. 1 is a cross sectional depiction of a mask used for SCALPEL. The mask 10 is made from a membrane layer 12 formed overlying a substrate 14. Typically, the membrane layer 12 is silicon nitride and the substrate 14 is a silicon wafer. On one side of the mask, scattering elements 16 are formed on the membrane layer 12. On the other side of the mask, portions of the membrane layer 12 and substrate 14 are removed to form struts.
FIG. 2 is an enlarged view of a portion of the mask 10 illustrated in FIG. 1. Portions of the membrane layer 12 lying between the struts 18 form the membranes 17. The scattering elements 16 are located over the membranes 17. The scattering elements 16 and struts 18 are formed on opposite sides of the mask 10. The struts 18 include remaining portions of the silicon wafer 14 and membrane layer 12 after a membrane formation step. The scattering elements 16 can be formed before or after the membranes 17 are formed.
One problem in manufacturing the SCALPEL mask is the formation of features (struts 18 and scattering elements 16) on alternate sides of the mask 10. Typically, the struts are first defined by the nitride layer on one side of the mask 10. During subsequent processes, scattering elements 16 are formed on the opposite side of the mask 10. The substrate 14 is then etched to finish the formation of the struts 18. Alternately processing the mask 10 on different sides of the mask 10 increases its potential for introducing handling and processing induced defects. Handling is generally performed using handling devices, such as tweezers, vacuum wands, or automated equipment. However, it should be noted that during transport, handling, and processing, these handling devices should not contact the scattering elements 16 or the struts 18. Processing sequences that form the scattering elements 16 before completely forming the struts 18 have a potential to create a pattern distortion of the scattering elements 16 that occurs due to etching the substrate 14. The exact mask fabrication sequence, patterning the scatterer before membrane formation or patterning the scatterer after membrane formation is still a matter of debate. However, processing and handling considerations must be addressed in order to accurately produce the scattering elements 16 on the SCALPEL mask 10.